Electrochemical synthesis of esters



United States Patent 3,326,784 ELECTROCHEMICAL SYNTHESIS OF ESTERS William J. Koehl, Jr., Yardley, Pa., assignor to Mobil Oil Corporation, a corporation of New York No Drawing. Filed Sept. 6, 1963, Ser. No. 307,000 9 Claims. (Ci. 204--80) This invention relates to the preparation of organic esters, particularly to the electrochemical synthesis of the same from carboxylic acids, and provides a simple straightforward method for making them in good yield.

The invention is advantageous in using only a carboxylic acid as the reactant, representing a simplification over chemical esten'fication reactions which require an alcohol to be reacted with an acid in the presence of a catalyst. The invention is thus useful in cases where the desired alcohol is not readily available. While esters have been identified in some electrochemical processes, the method provides increased yields over such known processes, in all or most of which the esters are regarded as by-products; thus, in some cases the method aifords ester yields of 20 to 80% or more as against 2 to in the known processes.

Briefly, the invention comprises electrolyzing an aqueous solution of an aliphatic carboxylic acid by passing current from an anode of carbon to a cathode immersed in the solution under the conditions herein described to produce an ester at the anode, and recovering the same. The invention makes use of a novel combination of operating conditions, including the use of anodes of carbon, preferably graphite. The ester product is the result of an anodic oxidation of the carboxylic acid.

Considering the invention in detail, the aqueous solution that is electrolyzed may comprise broadly, weight basis, about 5 or to 90% of the free carboxylic acid, about 5 to 25% of an alkali metal salt of the carboxylic acid, and the balance water, which may run from 5 to 90%. As indicated, the salt is a salt of the same free acid. For C-2 and C-3 acids, larger concentrations are desirable, going from 50 to 80 or 90%, with the salt about 5 to 10% and water 5 to 10%. For acids above C-4, preferred concentrations are lower, say 10 to 40 or 50%, the salt is 10 to 25%, and water is 50 to 80%.

Suitable acids are the C-2 to C6, preferably C-2 to C-5, acids such as acetic, propionic, butanoic and pentanoic and their isomers, and the various hexanoic acids.

Other useful acids are the C-7, C-8, C-9, and C-10 acids, although their solubility becomes progressively lower as the molecular weight increases; in these instances, use of larger quantities of the acid salts and of water tends to increase the solubility, so that in consequence the electrolyzing bath may contain smaller amounts of free acid.

It will be understood that thepresence of excess acid in the solution, even though it is undissolved' therein, is permissible. It simply remains until the dissolved acid is electrolyzed or used up in theelectrolyzing process, at which time it dissolves to replace the electrolyzed acid and is itself used up. It is also contemplated that the process may be continuously carried out, with acid being continuously introduced to the solution at a rate equal to, or even slightly greater than, the rate at which it is electrochemically reacted.

It may be noted that the carboxylic acid salt may be added per se to the solution or formed in situ as by addition of a base. like KOH or NaOH and reaction of the latter with part of the free carboxylic acid.

The pH of the electrolyte solution may initially be on the acid side, or neutral, but preferably is on the acid side, and suitably may range from a pH of 4 to 6, more broadly from 3 to 7. It is convenient in batchprocesses to electrolyze the acid solution until a pH of 7 is reached,

and then to collect products or replenish the solution.

The anode is of carbon, particularly graphite. Good results are obtainable from the following illustrative materials:

TABLE A.ANODE CARBON Apparent Real Pore Identity Density, Density, Volume,

g./cc. g./cc. cc./g.

The cathode may be carbon or graphite or any inert metal such as copper, stainless steel, platinum, silver, nickel, lead, etc. Forms of the electrodes are conventional.

The current density is desirably maintained in a narrow range, 0.04 to 0.06 amp/sq. cm., to favor an improved yield, although a wider range is operable, say 0.01 to 0.4 amp./ sq. cm. Applied voltage is supplied by any suitable DC source. 7

Room temperatures are preferred, e.g., 20 to 30 C., and somewhat higher temperatures are useful, but above about 50 C. the yield of ester product is adversely affected.

The current efiiciencies are in the range of about 10 to about 80%, being higher for the lower starting acids and vice versa. If desired, a diaphragm of conventional material may be used to separate the cathode from the anode in order to prevent possible reaction of the products formed at one electrode with those at the other. Agitation is desirable but can be omitted.

As indicated, the ester is formed at the anode by anodic oxidation. The anolyte may also be distilled or fractionated to recover the ester product, or it may be recovered by conventional extraction with a conventional solvent such as ether. Liquid by-products are recoverable by distillation, and gaseous by-products by conventional water displacement methods and equipment.

Ester products of the formula RCOOR' are produced, wherein R and R are alkyl groups, and wherein the moiety RCOO- is the same as in the starting acid RCOOH. The alkyl moiety -R' pionic, the ester formed therefrom is ethyl propionate.

It is also found that a starting acid having at least 4 carbon atoms yields a mixture of esters, normal and secondary, the normal being characterized by having a straight chain alkyl moiety R', and the secondary by having a branched chain R. Further, such mixture is characterized by having 2 to 3 times more secondary than normal ester.

Yields of ester product are variable, as might be expected from the decreasing water solubility of the higher starting acids, but they exceed the by-product quantities from conventional electrochemical processes, i.e., those processes in which esters were detected as by-prodnets and in which platinum anodes were used. With the lower acids the yields are quite good, propionic acid giving nearly 20% of ethyl propionate as against 5% for the conventional by-product process, and acetic acid giving over 80% of methyl acetate versus 2% for the by-product process. Yields are based on electricity. For n-butanoic acid, a yield of about 12% was obtained, which is superior over the by-product process.

As known, these organic esters have wide utility as solvents and plasticizers and in a variety of applications.

Besides aliphatic monobasic acids, aryl-substituted aliphatic carboxylic acids are capable of being electrolyzed, yielding esters, suitable acids including phenylacetic and phenylpropionic.

has one less carbon atom than the starting acid. Thus, if the starting acid is pro- The invention may be illustrated by the following examples.

EXAMPLE 1 Acetic acid was electrolyzed under a variety of conditions, including different cells, anodes, concentrations, current densities, and temperatures. A description of the cells and carbon anodes follows, after which the conditions and results are summarized in Table 1.

Cell A.-This cell consisted of a one liter vessel fitted with a stirrer, thermometer, reflux condenser, cooling bath and an electrode assembly. The electrode assembly was made of four 1 x 32 x 115 mm. copper plates and three 6.5 x 32 x 115 mm. carbon plates which were stacked alternately and held together by two Teflon rods which passed through all seven pieces. Teflon washers on the rods maintained 4 mm. spaces between the plates. This assembly was mounted vertically in the cell. The carbon plates were connected together as the anode and the copper plates were connected together as the cathode. The carbon plates were cut from the materials described in Table A.

Cell B.This cell consisted of a cylindrical glass vessel 5.0 cm. in diameter and cm. in height fitted with a reflux condenser and a thermometer. The cathode was a copper cylinder 3.0 cm. in diameter and 5.0 cm. in height which was concentric with a A inch diameter carbon rod. The carbon rods were National Carbon Company Intens-Arc welding rods which were cleaned in hot nitric acid before use. Lead dioxide anodes were also used in this cell. These were prepared by plating lead dioxide from a lead nitrate solution onto A1 inch carbon rods. The contents of cell B were stirred either with a magnetic stirrer, or when the lead dioxide anode was used by rotating the anode at 2500 r.p.m.

Cell C.-This cell was a 3.0 cm. diameter glass cylinder of about 100 ml. capacity fitted with a reflux condenser, thermometer, magnetic stirrer and two 1.5 x 3.6 cm. platinum foil electrodes mounted parallel and 0.6 cm. apart. I

In each run listed in Table 1 below, glacial acetic acid was used. Besides the liquid products noted, gaseous products were obtained including hydrogen, methane, ethane, carbon dioxide, and oxygen. The liquid product mixtures were obtained by ether extraction of the cell liquid and were analyzed by gas chromatography of a 10 ft. silicone gum rubber on Chromosorb column in a F and M Model 500 temperature programmed gas chromatograph. Calibration was done with known pure compounds. Identification of some products was confirmed by infrared spectroscopy and refractive index measurements on samples isolated by fractional distillation or by preparative gas chromatography on an Autochrome 200 chromatograph.

u Each solution was 0.7 to 0.8 M in anhydrous sodium acetate.

b After 12 amp. hr., 50 ml. added.

a In most of the electrolyses the applied voltage was increased as the reaction proceeded in order to maintain a constant current; consequently, the temperature also increased.

4 The yield of methyl acetate with cell A was 37 g. Distillation of the cell solution gave 35 g. of this ester.

EXAMPLE 2 A mixture of 450 ml. propionic acid, 30 g. potassium hydroxide and 30 ml. water was electrolyzed at 21 KC carbon anode in cell A at a current density of 0.055 amp/cm. and at 31:1" C. Analysis of the cell liquid showed that 6.7 g. of ethyl propionate formed, corresponding to a 19% yield, electricity basis. Ethylene was found among the gaseous products in 70% yield, based on electricity. A typical gas mixture from the electrolysis had the composition 36.0 mole percent hydrogen, 25.4 mole percent ethylene and 37.3 mole percent carbon dioxide. Traces of ethane, propane and carbon monoxide were also found, but no butane.

These data can be compared with a typical electrolysis of propionic acid at a platinum anode from which butane was obtained in 8% yield, ethylene 66%, and ethyl propionate 5%.

EXAMPLE 3 A solution of 60 g. of potassium hydroxide and 250 ml. of n-butanoic acid in 250 ml. of water was electrolyzed in Cell A with carbon anode KC at a temperature of 21 to 26 C., using a current of 40:0.2 amp. for 8 hours at 3 to 8 volts. The current density was 0.47 amp./ sq. cm. By neutralization of the cell solution with sodium bicarbonate and extraction with ether, about 9.0 g. of a butyrate fraction was obtained, corresponding to a 12% yield, electricity basis. This fraction comprised 32% propyl butyrate and 63% isopropyl butyrate.

In other runs, n-pentanoic acid was electrolyzed, using carbon anodes, with ester yields of about 10%, as against only about 4% in a conventional process where platinum anodes were employed. a

As may be apparent, the invention provides a convenient method for producing good yields of low molecular weight esters of the type RCOOR, where R and R are alkyls having the same number of carbons, starting from readily available carboxylic acids. The only reactant is the starting acid; and the only other chemical used is an acid salt, which may also be formed in situ from the acid and a conventional caustic. Salts of other carboxylic acids could be employed, but their use results in a decreased yield of desired ester.-

Some of the by-products of the process are valuable, including olefins and hydrogen.

While aqueous solutions of the electrolyte are preferred, it is possible to employ non-aqueous solutions. The usual objections to the latter is that they are poor conductors, but they do have an advantage in that the solubility, particularly of the higher carboxylic acids, may be increased if the latter are dissolved in a nonaqueous inert electrolyte, of which suitable examples include tetrahydrofuran, acetonitrile, dimethyl sulfoxide, acetone, dioxane, methanol, ethylene glycol, etc.

It will be understood that the invention is capable of obvious variations without departing from its scope.

In the light of the foregoing description, the following is claimed.

I claim:

1. A process for preparing methyl acetate from acetic acid comprising passing current from an anode of graphite to a cathode through an aqueous solution having a pH of 4 to 6 and containing, weight basis, up to 9 0% of acetic acid, 5 to 10% of an alkali metal salt of said acid, and 5 to 10% of water at a temperature of 20 to 30 C. and a current density in the range of 0.04 to 0.06 amp/sq. cm., producing said ester at the anode in a yield of at least electricity basis, and recovering the same.

2. A process for preparing a low molecular weight ester of the formula RCOOR' wherein R and R are alkyl groups having the same number of carbons, and wherein said alkyl groups have 1 to 2 carbons, which comprises passing current from an anode of graphite to a cathode through an aqueous solution having a pH of 4 to 6 and containing, weight basis, up to 90% of a C-2 to C-3 aliphatic carboxylic monobasic acid, 5 to of an alkali metal salt of said carboxylic acid, and 5 to 10% of water at a temperature of 20 to 30 C. and a current density in the range of 0.04 to 0.06 amp/sq. cm., producing said ester at the anode in a yield of about 20 to 80%, electricity basis, and recovering the same.

3. A process for preparing a low molecular weight ester of the formula RCOOR wherein R and R are alkyl groups having the same number of carbons, and wherein said alkyl groups have 3 to 5 carbons, which comprises passing current from an anode of graphite to a cathode through an aqueous solution having a pH of 4 to 6 and containing, weight basis, 10 to 40% of a C-4 to C-6 aliphatic carboxylic monobasic acid, 10 to 25% of an alkali metal salt of said carboxylic acid, and 50 to 80% of water at a temperature of 20 to 30 C. and a current density in the range of 0.04 to 0.06 amp./ sq. cm., producing said ester at the anode, and recovering the same.

4. A process for preparing a low molecular weight ester of the formula RCOOR wherein R and R are alkyl groups having the same number of carbons, and wherein said alkyl groups have 1 to 5 carbons, which comprises passing current from an anode of graphite to a cathode through an aqueous solution having a pH of 3 to 7 and containing, weight basis, 5 to 90% of a C-2 to C-6 aliphatic carboxylic monobasic acid, 5 to 25 of an alkali metal salt of said carboxylic acid, and 5 to 90% of Water at a temperature below about 50 C. and a current density in the range of 0.01 to 0.4 amp./ sq. cm., producing said ester at the anode, and recovering the same.

5. A process for preparing a low molecular weight ester from a free aliphatic monobasic carboxylic acid having 2 to 6 carbon atoms wherein said ester is characterized by having an alkyl moiety containing one less carbon than said acid, and wherein said acid is the sole reactant, which comprises passing current from an anode of graphite to a cathode through an aqueous solution having a pH of 3 to 7 and containing, weight basis, 5 to 90% of said acid, 5 to 25% of an alkali metal salt of a carboxylic acid, and 5 to 90% of water at a temperature below about C. and a current density in the range of 0.01 to 0.4 amp/sq. cm., carrying out said electrolysis in the absence of added inert inorganic anions, producing said ester at the anode in a yield of at least 20%, electricity basis, and recovering the same.

6-. The method of claim 5 wherein said ester is continuously removed from said solution and said free acid is continuously introduced thereto.

7. The method of claim 5 wherein a portion of said free acid is initially present in said solution in undissolved form and wherein said portion dissolves as free acid and is electrochemically reacted.

8. The method of claim 5 wherein said graphite is characterized by having a real density of at least 2.0 g./ cc. and a pore volume of less than 0.5 cc./ g.

9. A process for preparing a low molecular weight ester from a free aliphatic monobasic carboxylic acid having 2 to 6 carbon atoms wherein said ester is characterized by having an alkyl moiety containing one less carbon than said acid, which comprises passing current from an anode of graphite to a cathode through a solution containing, weight basis, 5 to of said acid and 5 to 25% of an alkali metal salt of a carboxylic acid at a temperature below about 50 C. and a current density in the range of 0.01 to 0.4 amp/sq. cm.

Brockman, C. J.: Electro-Organic Chemistry, 1926, New York, pages 23 to 30.

JOHN H. MACK, Primary Examiner. H. M. FLOURNOY, Assistant Examiner. 

1. A PROCESS FOR PREPARING METHYL ACETATE FROM ACETIC ACID COMPRISING PASSING CURRENT FROM AN ANODE OF GRAPHITE TO A CATHODE THROUGH AN AQUEOUS SOLUTION HAVING A PH OF 4 TO 6 AND CONTAINING, WEIGHT BASIS, UP TO 90% OF ACETIC ACID, 5 TO 10% OF AN ALKALI METAL SALT OF SAID ACID, AND 5 TO 10% OF WATER AT A TEMPERATURE OF 20 TO 30*C. AND A CURRENT DENSITY IN THE RANGE OF 0.04 TO 0.06 AMP./SQ. CM., PRODUCING SAID ESTER AT THE ANODE IN A YEILD OF AT LEAST 80%, ELECTRICITY BASIS, AND RECOVERING THE SAME. 